Silicon wafers or sheets may be used in, for example, the integrated circuit or solar cell industry. Demand for solar cells continues to increase as the demand for renewable energy sources increases. One major cost in the solar cell industry is the wafer or sheet used to make solar cells. Reductions in cost to the wafers or sheets may reduce the cost of solar cells and make this renewable energy technology more prevalent. One method that has been investigated to lower the cost of materials for solar cells is the horizontal pulling of thin silicon ribbons from a melt that cool and solidify into a crystalline sheet, referred to herein as horizontal sheet growth.
In producing a silicon crystalline sheet by a horizontal sheet growth method known as the Floating Silicon Method (FSM), a useful component is a cooling device, which cooling device removes latent heat from the silicon crystalline sheet via the impingement of a cooling gas, for instance Helium, onto the ribbon surface. The cooling device may be arranged as a long slit or channel through which channel the cooling gas flows and is directed onto the ribbon surface. The cooling device is positioned a few mm above the surface of the crystallizing sheet during growth, and may be moved away from the surface when the ribbon (crystallizing sheet) is not being grown. The ribbon is grown underneath the slit and pulled at a constant rate to one side in order to grow continuous lengths of ribbon.
In particular, during horizontal sheet growth of silicon, the cooling device may employ a cold block, where the cold block may be used to crystallize a horizontal sheet from a silicon melt. Due to the crystalline structure of silicon, the leading edge of a single crystalline sheet, sometimes referred to as a “single-crystal ribbon,” or “silicon ribbon,” is defined by a (111) facet. Growing this faceted leading edge at a fast rate (>1 mm/s) entails the use of intense heat removal within a very narrow region at the leading edge of the single-crystal ribbon, where peak heat removal may be well over 100 W/cm2. In order to generate such high cooling rates, generating a vortex between a relatively hot single-crystal ribbon surface and the cooling device is useful. This vortex may be created by flowing gas at a rapid rate from a channel or passage in the cooling device, such as a cold block, toward the melt surface of the silicon melt (molten Si). Such a vortex may disadvantageously also carry SiO from the molten Si to a cold surface of the cooling device, where the SiO condenses, forming SiOx particulates. Accordingly, the growth of a crystalline sheet of silicon may entail a balance between maintaining the high heat transfer from the solidifying silicon ribbon to the cooling device, while at the same time avoiding SiOx deposition.
Another challenge related to silicon horizontal sheet growth is the need to avoid SiOx formation in a furnace before sheet growth. To form a silicon melt, a Si melt-in process may be employed prior to forming a crystalline sheet from the silicon melt. During the Si melt-in process, a cooling device may be raised to a separation from the melt surface of, for example, greater than 1 cm from the melt surface. During this melt-in time, the concentration of SiO may be high due to the high temperature employed during melt-in (well above the Si melt temperature), as well as the possibility of silicon feedstock having SiO2 on the large effective surface area of the silicon. Preventing this SiO from condensing on all “cold” surfaces (where a “cold surface” may be defined for the purposes of illustration in the context of silicon ribbon growth as a surface having a temperature less than 1250 C) of the cooling device is especially useful, even when the cooling device is disposed at a separation >1 cm above the melt. Notably, in a cooling device including a water cooled cold block, there is an inherent temperature gradient between the surface of the water-cooled cold block (providing the heat removal), and the outside walls of the cooling device, where the outside walls are heated to above Si melt temperature (1412° C.). This temperature gradient will result in exposed areas of the cooling device that constitute such “cold” surface, at a temperature less than 1250° C., and therefore susceptible to SiO condensation. There is therefore a need to prevent ambient furnace gas from reaching these “cold” surfaces.
Another challenge encountered in horizontal sheet growth is the limited visibility by an operator or camera to the horizontal sheet/melt interface in the area underneath a cooling zone created by the cooling device. Notably, for proper growth of a silicon sheet, the lower surface of the cooling device may be maintained at a distance less than −3 mm from the Si melt surface, in order to maximize cooling, making visible access to the leading edge of the ribbon difficult.
With respect to these and other considerations the present improvements are provided.